Take the generator output directory into account when trying to invoke gyp

Otherwise, the gyp binary won't be found when trying to regenerator the output
files

R=thakis@chromium.org
TEST=test/hello/gyptest-regyp-output.py

Review URL: https://codereview.chromium.org/17847002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1656 78cadc50-ecff-11dd-a971-7dbc132099af